This application is based on application No. MI99A 001151 filed in Italy, the content of which is incorporated hereinto by reference.
The present invention relates to an apparatus and a method for heatsetting a knitted fabric in tubular form.
Circular knitted fabrics, depending on the circumstances, may be required to undergo successive processing operations for the preparation, dyeing, finishing and making-up thereof both in tubular form, as they descend from the circular knitting machine, and in an open form, obtained by cutting the tubular fabric along one of its side edges.
Generally, before undergoing wet treatments such as, for example, dyeing, the fabric is subjected, depending on the type or types of fibre which form it, to a heatsetting operation.
Heatsetting is performed, in particular, on fabrics composed of or partly containing synthetic fibres, such as thermoplastic fibres. The aim of heatsetting is to fix the dimensions and the flat state of the surfaces of the fabrics for providing them with stability and not cause permanent creases or distortions during the course of processing in bleaching and dyeing machines.
With the heatsetting machines which are currently available it is not possible to differentiate between the processing of fabrics with an orthogonal pattern, i.e. of the weft/warp type, and the processing of knitted fabrics in general, and it also necessary for the knitted fabrics to be in an open form.
This results in an increase in the duration of the processing cycle due to cutting and opening of the tubular fabric for heatsetting and, sometimes, re-stitching of the fabric in a tubular form for the bleaching/dyeing operation, followed by reopening for the finishing and making-up operation.
Moreover, these machines are unable to perform the heatsetting of circular knitted fabrics which must be completely processed in tubular form either for technical reasons or for cost-related reasons or because of market requirements.
On the other hand, processing in tubular form without heatsetting results in considerable risks from the point of view of quality and is only performed in exceptional cases.
Hitherto various attempts have been made to develop specific machines for heatsetting fabrics in tubular form. However, the results obtained are somewhat unsatisfactory. The known machines are characterized essentially by the method of conveying and guiding the tubular fabric and by the system for transferring heat to the fabric.
In some machines, the tubular fabric is guided by means of a flat expansion device and is fed in a flattened form between the surface of a heated steel cylinder and an endless felt belt. The main drawbacks of these machines consist in the squashing of the side edges of the tubular fabric and a non-uniform heat exchange on its two surfaces.
In other machines, the tubular fabric is guided by means of a flat expansion device which accompanies it in a flattened form through a horizontal or vertical chamber until it emerges therefrom. Inside the chamber, the two surfaces of the tubular fabric are acted on by flows of hot air. In this case, squashing of the side edges of the fabric does not occur, but, during conveying, the fabric is subject to undesirable elongation. Moreover, the heat exchange on the two surfaces of the fabric is inadequate.
An object of the present invention is to avoid the drawbacks and overcome the limitations of the known machines for heatsetting fabrics in tubular form.
According to a first aspect thereof, the invention relates to an apparatus for heatsetting a knitted fabric in tubular form, comprising:
a) means for feeding said tubular fabric,
b) steam-treatment means designed to direct steam onto said tubular fabric,
c) first flat expansion means capable of extending said tubular fabric in a substantially flattened form, with two superimposed surfaces, and causing said flattened tubular fabric to assume a preselected width,
d) conveying means capable of feeding said flattened tubular fabric,
e) heatsetting means capable of forming at least one air flow, at a preselected temperature, and associated with said conveying means, and
f) means for stabilizing said heatset tubular fabric, characterized in that it comprises:
g) rod-shaped guiding means associated with said conveying means and said heatsetting means, said rod-shaped means being capable of supporting internally said flattened tubular fabric along its side edges, owing to the action of said heatsetting means, while keeping the said side edges extended, leaving said superimposed surfaces free to be passed through by said air flow and keeping said width of said flattened tubular fabric unchanged.
Preferably, the apparatus also comprises second flat expansion means, said first flat expansion means and said second flat expansion means being located respectively upstream and downstream of said conveying means, said second flat expansion means being capable of keeping said tubular fabric in a substantially flattened form, with two superimposed surfaces, while maintaining said preselected width, said rod-shaped means being connected to said first and second flat expansion means for assuming a mutual distance, equal to said width of said flattened tubular fabric.
Advantageously, said heatsetting means are capable of directing said air flow towards said flattened tubular fabric for passing through and strike said two superimposed surfaces.
According to a preferred embodiment, said conveying means comprise a drum having a cylindrical side wall onto which said flattened tubular fabric is wound, said drum being capable of feeding said fabric with positive driving, without applying a longitudinal tension, said rod-shaped means being formed by two rods with a curvilinear cross-section and substantially annular form, associated with said drum for supporting internally said side edges of said tubular fabric wound onto said cylindrical side wall.
Advantageously, said two substantially annular rods are integral with said first and second flat expansion means for assuming said mutual distance, and slider means are associated with said two rods for keeping them at said mutual distance with respect to said cylindrical side wall.
Preferably, said cylindrical side wall of said drum is perforated and said heatsetting means form substantially radial air flows, at a preselected temperature, capable of passing through said perforated wall and passing through and striking said two superimposed surfaces of said flattened tubular fabric for performing said heatsetting while said tubular fabric is wound onto said perforated drum.
According to another embodiment, said conveying means comprise two conveyor belts which are facing each other and between which said flattened tubular fabric is arranged, said two conveyor belts being capable of feeding said fabric with positive driving, without applying a longitudinal tension, said rod-shaped means being formed by two substantially straight rods with a curvilinear cross-section, associated with said conveyor belts for supporting internally said side edges of said tubular fabric arranged between said conveyor belts.
Preferably, said two substantially straight rods are integral with said first and second flat expansion means for assuming said mutual distance.
Advantageously, said conveyor belts are perforated and said heatsetting means form opposing air flows, at a preselected temperature, capable of passing through said perforated belts and passing through and striking said two superimposed surfaces of said flattened tubular fabric for performing said heatsetting while said tubular fabric is arranged between said perforated belts.
According to a second aspect, the invention relates to a method for heatsetting a knitted fabric in tubular form, comprising the steps of:
i) treating said tubular fabric with steam,
ii) expanding said tubular fabric, extending it in a substantially flattened form, with two superimposed surfaces, and causing said flattened tubular fabric to assume a preselected width,
iii) conveying said flattened tubular fabric,
iv) heatsetting said flattened tubular fabric during conveying thereof, by means of at least one air flow, at a preselected temperature, and
v) stabilizing said heatset tubular fabric, characterized in that
vi) said flattened tubular fabric is supported internally along its side edges during said heatsetting, while keeping said side edges extended and leaving said superimposed surfaces free to be passed through by said air flow and keeping said width of said flattened tubular fabric unchanged.
Preferably, said heatsetting air flow passes through and strikes said two superimposed surfaces of said flattened tubular fabric.
Advantageously, said conveying of said flattened tubular fabric is performed with positive driving, without applying a longitudinal tension thereto.
The apparatus and the method according to the invention offer numerous advantages.
Principally, they allow the side edges of the tubular fabric to be supported internally, protecting them during the critical phase of heatsetting. The risks of adversely affecting and rendering unacceptable the quality of the processed tubular fabric are thus minimised.
With the apparatus and the method according to the invention, there is also a very high, efficient and uniform heat exchange between air and fabric because it is performed on both surfaces of the fabric, owing to the forced flow of air through the fabric.
Moreover, driving of the fabric is always positive in any zone of the fabric path and between one zone and another. In particular, the fabric is never driven with a longitudinal pulling force.
Therefore, the transverse and longitudinal dimensions of the fabric are defined and maintained in each zone of the apparatus.
All this improves the quality of the product.
The conditions of:
protection of the side edges of the tubular fabric,
efficient and uniform transfer of heat to the two surfaces of the tubular fabric, and
positive driving of the tubular fabric without longitudinal pulling, resulting in the transverse and longitudinal dimensions being maintained in each zone of the apparatus, are never obtained in known machines.